page waitqueue: always add new entries at the end

Commit 3510ca20 ("Minor page waitqueue cleanups") made the page
queue code always add new waiters to the back of the queue, which helps
upcoming patches to batch the wakeups for some horrid loads where the
wait queues grow to thousands of entries.

However, I forgot about the nasrt add_page_wait_queue() special case
code that is only used by the cachefiles code.  That one still continued
to add the new wait queue entries at the beginning of the list.

Fix it, because any sane batched wakeup will require that we don't
suddenly start getting new entries at the beginning of the list that we
already handled in a previous batch.

[ The current code always does the whole list while holding the lock, so
  wait queue ordering doesn't matter for correctness, but even then it's
  better to add later entries at the end from a fairness standpoint ]
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -1041,7 +1041,7 @@ void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter)
 	unsigned long flags;
 
 	spin_lock_irqsave(&q->lock, flags);
-	__add_wait_queue(q, waiter);
+	__add_wait_queue_entry_tail(q, waiter);
 	SetPageWaiters(page);
 	spin_unlock_irqrestore(&q->lock, flags);
 }